1. Field of the Invention
This invention relates to an objective lens for photographing, and in particular to an objective lens capable of short distance (close-up) photographing which has a high performance in all photographing ranges from an infinity object to a short distance object for which the photographing magnification is one-to-one magnification.
2. Related Background Art
As an objective lens system of this type, there is known an objective lens of three-group construction having, in succession from the object side, a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group disposed rearwardly of the second lens group and having a negative refractive power and functioning as a rear converter. In the focusing system from infinity photographing to one-to-one magnification photographing, there have heretofore been proposed various focusing systems such as a system in which the first lens group and the second lens group are axially moved together toward the object side, and a system in which the spacing between the first lens group and the second lens group is enlarged while the first lens group and the second lens group are axially moved toward the object side.
However, these focusing systems have suffered from the disadvantage that the amount of movement of the first lens group becomes great and the variation in the full length from infinity photographing to one-to-one magnification photographing is great. As an objective lens system for overcoming such disadvantage, the system disclosed, for example, in Japanese Laid-Open Patent Application No. 62-22711 is already known. In this publication, use is made of a system whereby during the focusing from infinity photographing to one-to-one magnification photographing, the first lens group and the second lens group as previously described are moved together toward the object side and the magnification of the third lens group is made great to thereby make the amount of movement during short distance focusing small and make the variation in the full length of the lens system small.
However, in the lens system disclosed in this publication, the magnification of the negative third lens group is basically made great and therefore, the negative refractive power of this third lens group must be strengthened and therefore, the balance of Petzval sum is lost and becomes excessively negative, and correction of curvature of image field, astigmatism, etc. becomes difficult, and this is not preferable.
Also, it is unavoidable to strengthen the positive refractive power of the composite lens system provided by the first and second lens groups, and spherical aberration occurs greatly over the infinity photographing condition to the close distance photographing condition, and this leads to the problem that the lens performance is remarkably deteriorated.